The present invention relates to an optical data link for providing high-speed data communication between electronic data processing equipment at geographically separate locations to permit the full system integration of the electronic data processing equipment.
The trend in configuring electronic data processing (EDP) systems is toward dispersing the access to EDP equipment. For dispersed EDP equipment located in the same building, interconnecting the various physically separate units can usually be accomplished by cabling. However, as system architectures expand to incorporate EDP equipment dispersed in separate buildings, transmitting data becomes more complex. The installation of dedicated inter-building cabling, even where technically feasible, may be economically or physically impractical. Inter-building data communication via telephone transmission facilities is costly and can reduce system flexibility and integration because of the inherent limitations on teleprocessing data transfer rates.
One solution to the problem of inter-building, high-data rate data communication is to use a communication link to transmit data over a modulated light beam. Optical communication systems have been used for many years--an example of an early optical data system is shown in U.S. Pat. No. 3,705,986 to Sanders. The trend toward dispersed data processing has given new impetus to the development of such systems for high-speed digital data communication.
While optical data links have been devised using coherent laser light, the use of non-coherent light is advantageous in terms of cost and avoiding governmental regulatory restrictions. However, the use of non-coherent light imposes severe gain control and attendant signal reproduction requirements, particularly at current multiple megabit data transfer rates. To illustrate, for a range of one to two miles, an optical data link must provide signal amplification for a data signal whose amplitude may vary over a range of one to one million while the signal-to-noise ratio of the received modulated optical signal is typically in a range from 1/1,000 to 1/10,000. Signal processing is particularly complicated during daylight hours by the presence of background noise having a DC component due to sun light and AC components due primarily to atmospheric scintillation.
An optical data link can use two different attenuation techniques to provide gain control. First, electronic attenuation control can be provided in the gain stage(s) of the receiver channel electronics to regulate the amount of signal amplification. Second, the amount of optical energy impinging upon a photoreceptor at the receive-end of the link can be regulated by mechanical means (such as by closing an iris), thereby regulating the amount of optical energy available to be converted to the electrical signals supplied to the gain stage(s). While electronic attenuation is preferred, dynamic range requirements preclude the sole reliance on electronic attenuation for providing gain control. Therefore, electronic attenuation must be supplemented by some means of regulating the amount of optical energy coupled to the receive-end photoreceptor. However, the amount of such optical attenuation is preferably minimized because optical attenuation reduces modulated signal level by a linear function while superimposed noise is reduced only by a square root function; so that the signal-to-noise ratio is adversely affected.
Optical energy attenuation unrelated to gain control can affect gain control and signal reproduction where an optical data link transceiver with plastic optical elements is subject to ambient temperature variations. Current optical data links typically use plastic Fresnel lenses both at the transmit-end for collimating the modulated light output from a radiant source and at the receive-end for focusing incident optical energy onto a photoreceptor; plastic Frensnel lenses are used because they are obtainable in large diameters at a fraction of the cost of a glass lens. However, when plastic lenses are used in an environment subject to significant temperature variation they are susceptible to temperature distortion that can significantly alter the lens focal length. For example, typical ambient temperature variations can alter the focal length for a 10-inch diameter plastic Fresnel lens by as much as 1/2 inch.
Such temperature-induced focal length distortion affects both optical transmission and reception. At the transmit-end, a temperature-induced translation of the lens focal point reduces the amount of optical energy from the radiant source (located at the nominal lens focal point) that is collimated for transmission over the optical data link. At the receive-end, focal point translation attenuates the optical energy coupled to a photoreceptor (located at the nominal lens focal point). Since system flexibility requires that the optical data link design accommodate those applications where exterior (balcony or rooftop) transceiver mounting is required, some means of correcting temperature-induced focal length distortion would be advantageous.